Voltage regulators are known that can convert from input voltages above, below, or equal to the controlled output voltage, respectively performing buck mode regulation, boost mode regulation, or buck-boost mode regulation. Regulator architecture typically is provided for power supplies for automotive applications, lap-top computers, telecom equipment and distributed power systems. A known “four-switch” buck-boost converter is described in an October 2001 datasheet for the LTC3440 “Micro-power Synchronous Buck-Boost DC/DC Converter” integrated circuit manufactured by Linear Technology Corporation. Two of the four switches are connected to the input side of an inductor, the other switches connected to the output side. In accordance with the level of voltage output to be controlled and the level of voltage input, the regulator has the capability of assuming a plurality of operation states in which the switches variously are sequentially activated or deactivated, to connect the inductor to the input, the output, and/or a common ground connection.
FIG. 1 is a simplified schematic diagram of a four switch regulator, such as the LTC3440. Four controllable switches are represented by blocks labeled A-D. Inductor 10 is coupled at one end to input voltage VIN via “A” switch 12 and to a common terminal via “B” switch 14. At its other end, inductor 10 is coupled to the output VOUT via “D” switch 16 and to the common terminal via “C” switch 18. During normal load buck mode operation, the inductor is repetitively switched between an “AD” charging cycle, in which switches 12 and 16 are closed and switches 14 and 18 are open, and an “BD” discharging cycle in which switches 14 and 16 are closed and switches 12 and 18 are open. This mode maintains VOUT at a lower level than VIN. During normal load boost mode operation, the inductor is repetitively switched between an “AC” charging cycle, in which switches 12 and 18 are closed and switches 14 and 16 are open, and the AD cycle in which charge is transferred to the output. This mode maintains VOUT at a higher level than VIN. During normal load buck-boost mode operation, the inductor typically is repetitively switched among three cycles, the AC charging cycle, the AD charge transfer cycle, and the BD discharge cycle. This mode maintains VOUT at or near the level of VIN.
In many portable systems, when the output load is light and the output voltage is at its regulation voltage, switching regulators are controlled to go into a power saving burst mode operation. An output capacitor allows shut off of all unnecessary functions to significantly reduce quiescent current. This state is commonly called a “sleep” state. When output voltage drifts lower to a programmed level below the regulation level, the regulator “wakes up” and delivers a burst of energy to the output capacitor until the output voltage is back to regulation voltage and returns to the sleep state. The intermittent cycling repeats until the burst mode terminates in response to increased output load conditions.
The four switch converter architecture suffers from reduced efficiency accruing from switching losses when all four switches are operational. The need thus exists for improving the efficiency of such converters.